You didn't need to explain your absence, but I think people who got to know you will appreciate you decided to do it.
In these cases there is not much to say, apart from the usual "I'm sorry" kind of comment, which doesn't mean I'm not actually sorry for you. It means it's probably a useless thing to say, since it will not bring you comfort at all.
I also had some family issues in the past three years: my father, who was already a not healthy person and who was already battling against gradually increasing blindness, had two strokes that left him partly invalidated - not on a physical, but mainly on a mental level - and my mother underwent surgical cancer removal and months of chemotherapy. It's been hard, and there's still high chance of the same conditions returning in the next future. Other people close to me also had their dose of misfortunes. Everybody was in continuous state of tension, everybody was trying to grab some helping hand without having to leave the secure shelter of despair. In cases where family members have to face a serious illness or situation, you realize you have no time or will to keep yourself interested with many activities. You also think, maybe on a less conscious level, people have lives you don't know anything about, and as you don't know about them, they don't know about you. But trying to explain everytime, to tell somebody you don't feel like being your usual self is also something you don't feel like doing. you end being more and more isolated, and if this seems good from many points of view, because you feel partly justified for not getting involved in anything outside your private life, it also can be very difficult, because, yes, we all need a minute or two to hear something reassuring, even if also a banal word from a complete stranger. We all need to know everything's going to be better, that there is sanity behind the madness and cruelty of illness, behind the deception and the anguish it constantly generates, being this a lie or a truth. It takes time to recover from anything, and I think persons around diseased or ill people are the most unfortunate, after all. Because they even feel deprived of their right to have a life that can be damaged or changed forever, because sometimes they are also stripped of their more natural and best qualities without the possibility of being excused.
You said you've changed a lot. It would be hard to believe one can stay the same and go through life completely unaffected in a situation like yours. I'm sure you are a valuable person and you will be doing ok, both in moments when things are going better and in moments when everything seems impossible. I'm not very close to you, but you've always looked like a person one can count on. I'm sure your mother knows it in her times of despair. I'm sure she doesn't ever forget that. Being ill implies also a dose of broken promises and cheating. But feeling you have somebody around who cares, who's willing to do whatever, that's a consciousness that is always with you. In moments of rage and selfishness you can deny it, you can say you're completely alone, that nobody cares and nobody matters, but it still is part of yourself. I don't know, but I suppose your mother knows it too. Your role is very important in her life, given her conditions. But don't give everything you have away, always keep a little of hope and pleasure for things you like with you. Always. Changing is one thing, losing everything that is important for you and your well-being and giving up is another. Love and affection don't have to be your own annihilation. This can never be.
I wish you all the best and I'm looking forward to some more of your posting, when you feel like it
